1. Field of the Invention
This invention relates to the field of acousto-optics and array antennas. In particular, the invention relates to acousto-optic beam deflectors and acousto-optic modulators and to their use in the control of the array antenna pattern.
2. Description of the Related Art
The interaction between light and sound has been under active investigation. Robert Adler, describes an acousto-optic beam deflector using a Bragg cell deflector in a May, 1967 article in IEEE Spectrum entitled "Interaction Between Light and Sound". Following this, the development of acousto-optic beam deflectors has progressed rapidly. A conventional acousto-optic beam deflector comprises a piece of isotropic optic material, such as glass, or non-isotropic material, such as tellurium dioxide,in which a travelling longitudinal or shear acoustic wave has been excited. The travelling acoustic wave modulates the index of refraction of the glass through a stress optic effect. An incident light beam is then deflected by the travelling phase grating produced by the travelling acoustic wave.
FIG. 1 illustrates a conventional beam deflector. A point light source 4 and collimating lens 6 form laterally coherent collimated light beam 7 parallel to axis 2. Coherent light beam 7 passes through Bragg cell 8 as coherent light beam 9a. Light beam 9a is focused by Fourier lens 10 onto focal plane 12 at point 12a. When Bragg cell 8 is driven by acoustic source 8a the refractive index of the material from which Bragg cell 8 is made is modulated so that coherent beam 7 is partially defracted through an angle as light beam 9b. Light beam 9b is focused by Fourier lens 10 on focal plane 12 at point 12b. The amount of beam deflection varies according to the wavelength of the acoustic wave in Bragg cell 8, which in turn varies inversely with the frequency of acoustic source 8a. Therefore, a change in the frequency of acoustic source 8a will cause a change in the angle of deflection of the acousto-optic beam deflector.
A conventional application of the acousto-optic beam deflector of FIG. 1 is as a spectrum analyzer. An analyzer comprises an array of detector elements (not shown) disposed along Fourier plane 12 such that a separate detector is disposed at point 12a, point 12b and other points along Fourier plane 12. Light from light source 4 will be focused on one detector in the array disposed along Fourier plane 12 in accordance with the frequency of acoustic source 8a. In this way the frequency components that make up acoustic source 8a are determined.
It is also known that the acoustic wave modulates coherent beam 7 such that the frequency of beam 9b is decreased by the frequency of acoustic source 8a. However, if the travelling acoustic wave were initiated from the bottom of Bragg cell 8, as shown in the figure, then the frequency of beam 9b would be increased by the frequency of acoustic source 8a.
Thus, according to the prior art, acousto-optic beam deflection is always accompanied by frequency shift of the beam, and modulation of the beam is always accompanied by beam deflection.
Corporately fed phased array antennas and radars employing phased array antennas have been known in one form or another in the relevant arts. For example, U.S. Pat. No. 4,258,363 to Bodmar et al. describes a phased array radar system with an optical fiber feed. Beam steering of the radar beam is controlled by phase shifters in each TR module; however, there is no disclosure as to how the phase shifter controls are driven. The light generated in the laser diodes of the transmitting section and fed through optical fibers to the TR modules does not contain any encoded information as to beam forming or beam steering. Light generated in a laser diode within each TR module and modulated with the intermediate frequency signal generated in the mixer in each TR module has no coherence synchronization with corresponding lights from other TR modules. Accordingly, the additive combination of these lights in the fiber converging unit and in the combination of the outputs of the photodiodes may have unpredictable results due to constructive and destructive superpositioned phases.
U.S. Pat. No. 4,028,702 to Levine describes a fiber optic phased array antenna system for RF transmission comprising RF modulated optical carrier light carried on a plurality of optical fibers to a plurality of multi-channeled fiber optic delay lines. Each multi-channel fiber optic delay line produces from its input optical signal a plurality of optical signals, each signal corresponding to a unique phase delay. All optical signals corresponding to a unique phase delay are fed into a switchable transducer to demodulate a selected optic signal. A desired phase gradient across radiator elements is produced by proper selection of the discretely switched phase delay signals. This arrangement makes it difficult to match phase shifts produced by the plurality of multi-channel fiber optic delay lines across all elements of the phased array.
U.S. Pat. No. 4,814,773 to Wechsberg et al. describes a fiber optic feed network for a radar comprising a conventional RF phased beam forming network, an optical feed network and a plurality of TR modules forming an antenna. The antenna beam is steered according to the conventional beam forming network.
U.S. Pat. No. 4,965,603 to Hong et al. describes an optical beam forming network for controlling a RF phased array. The beam forming network comprises a spacial weight computation system, a temporal control system and an optical fiber bundle that distributes optical signals to photodetector elements corresponding to radiation elements.
U.S. Pat. No. 4,814,774 to Herczfeld describes an optically phased array system and method. The system comprises a plurality of optical fibers each of which carries a modulated optical signal into a TR element. Each optical fiber is wrapped once around a substantially disk-shaped piezoelectric crystal, wherein the circumference of the crystal is controlled by a voltage applied to the crystal according to a desired phase delay.
U.S. Pat. No. 4,620,193 to Heeks describes an optical phased array radar, and in particular, a means to adjust an optical path length of an optical link between a central processor and an antenna element. The means comprises a means to sense the relative phase of a reflected signal with respect to an incident signal and then adjust the optical path length according to the sensed phase difference.
U.S. Pat. No. 3,878,520 to Wright et al. describes an optically operated microwave phased-array antenna system comprising an optical phase processor and an optical to microwave converter for driving the phased array antenna. The optical phase processor comprises a reference optical link and a plurality of modulated optical links, each modulated optical link being modulated by an optical modulator. The optical modulator comprises a beam splitter, a quarter-wave plate circular polarizer, a polarization analyzer, two light gates, a quarter-wave retardation plate and an optical combiner to modulate the phase of the modulated optical links.
U.S. Pat. No. 4,864,312 to Huignard et al. describes a device for optical control of a beam-scanning antenna comprising an array of spatial modulators, each modulator adjusting the optical path length to control phase by controlling electro-reflectance or by controlling electrically controllable refractive index.
U.S. Pat. No. 4,885,589 to Edward et al. describes an optical distribution of signals and antenna returns in a phased array radar system. The system provides a modulated and an unmodulated light to a plurality of elemental TR modules, each module having an optical switch which is switched according to a transmit mode or a receive mode. When transmitting, the optical switch passed the modulated light to a detector where it is amplified and passed to antenna element. When receiving, the optical switch passes the unmodulated light to an optical modulator to be modulated according to a received signal from the antenna element. Each TR module has its own conventional microwave phase shifter.
U.S. Pat. No. 4,507,662 to Rothenberg et al. describes an optically coupled array antenna comprising a first antenna array having an energy exchanging relationship with free space, a space coupling region, a second antenna array having an energy coupling relationship with the space coupling region and correspondingly coupled to elements of the first array, a third antenna array having an energy coupling relationship with the space coupling region, and a means for providing elements of the third antenna array with a signal having a preselected phase and amplitude according to a distribution across the array.
U.S. Pat. No. 4,929,956 to Lee et al. describes an optical beam former for high frequency antenna arrays comprising a constrained lens comprising a first array of optical lenses mounted on a first concave surface for receiving and collecting light emanating from a point, a second array of lenses mounted on a second concave surface for emanating light and an array of optical fibers connecting lenses in the first array with the corresponding lenses in the second array such that light emitted from a point in omni direction and received by the first array of lenses will be re-emitted by the second array of lenses as a parallel beam of light. The direction of the beam corresponds to the position of the originating point of light.